Gas phase reactions for the production of olefin polymers are well known in the art. Such gas phase reactions are typically carried out by fluidized bed, stirred or paddle-type reaction systems, and the like, as described in, for example, U.S. Pat. Nos. 4,588,790, 3,256,263, 3,625,932, British Patent Nos. 1,248,951, 1,248,952, 1,248,953, and the like. As used herein, a "polyolefin" is meant to include homopolymers, copolymers, and terpolymers of alpha-olefins and may optionally contain dienes, aromatic compounds with vinyl unsaturation and/or carbon monoxide.
Generally, the alpha-olefin monomers have from 2 to 12 carbon atoms and typically include, but are not limited to, ethylene, propylene, butene-1, pentene-1, 4-methylpentene-1, hexene-1, styrene, and the like. Preferred dienes which may optionally be polymerized with the alpha-olefins are those which are non-conjugated. These non-conjugated diene monomers may be straight chain, branched chain or cyclic hydrocarbon dienes having from about 5 to about 15 carbon atoms. Dienes which are especially preferred include 1,4-hexadiene, 5-ethylidene-2-norbornene.
Preferred aromatic compounds having vinyl unsaturation which also may be optionally polymerized with the alpha-olefins include styrene and substituted styrene.
So too, Group VIII transition metal compounds may be utilized to copolymerize carbon monoxide and alpha-olefins to form alternating co-polymer.
A catalyst is usually required to cause polymerization of the one or more alpha-olefin monomers, and the optional dienes, to take place. Such catalysts may include, but are not limited to, coordinated anionic catalysts; cationic catalysts; free-radical catalysts; anionic catalysts, and the like.
As more fully described in, for example, U.S. Pat. Nos. 3,779,712; 3,876,602; and 3,023,203, such catalysts are generally introduced into the reaction zone as solid particulates in which the catalytically active material is impregnated onto an inert support typically made of alumina or silica, and the like, to form the useable catalyst. As used herein, the term "inert" modifying a particular material, be it a catalyst support or a solvent, etc., means that the material being referred to is non-deactivating in the reaction zone under the conditions of the gas phase polymerization reaction and is non-deactivating with the catalyst in or out of the reaction zone.
Those skilled in the art have long believed that for polymerization reactions, particularly gas phase polymerization reactions, it is necessary to provide the catalyst impregnated on an inert support so as to facilitate control of polymer particle size and thereby control of the product bulk density. See, for example, U.S. Pat. No. 5,057,475. In particular, those skilled in the art believe that the size of the supported particulate catalyst is determinative of the polymer particles that are produced during the reaction, i.e., the polymer particles are about 10 to 15 times greater than the size of the supported particulate catalyst. Consequently, those skilled in the art would expect that the use of a catalyst which was unsupported would produce undesirable results. Indeed, in published European Patent Application No. 0 232 595 B1, in discussing slurry polymerization reactions utilizing a homogeneous catalyst system, i.e., an unsupported catalyst, it is taught that a disadvantage of such a catalyst system is that the polymer product produced manifests a small particle size and low bulk density. Moreover, impregnating the catalytically active material on a support is believed by those skilled in the art to desirably dilute the active centers of the catalyst. This is believed to provide greater isolation of such active centers and expose more of such sites to the monomer so as to facilitate polymerization.
One of the disadvantages associated with supported catalysts which are conventionally used in gas phase polymerization reactions, however, is that the support material, such as the alumina, silica, and the like, remains behind in the polymer product as inorganic residual ash thereby increasing the overall impurity level of the polymer depending upon the amount of such impurity, some of the properties of the polymers may possibly be affected, such as film appearance rating, impact resistance, tear strength, and the like.
So too, by being impregnated on a support, the activity of the catalyst is generally influenced by the available exposed catalyst surface area that comes into contact with the reactants. This is typically a function, among other things, of the porosity and volume of the support that is being utilized. When a support fails to provide an adequate surface area to volume ratio, then the catalyst will not exhibit high activity.